Proteins are elements that play a part in building our cells. They are our main source of energy, necessary for all kinds of metabolic processes. Our skin, muscles, internal organs, hormones, enzymes, genes and all sorts of tissues are made of proteins.
Because of the major role they play, a diet deprived of proteins is likely to cause problems and various diseases. Food supplies us with animal proteins (meats, fishes, eggs and milk products) and proteins of vegetable origin (dried vegetables, cereals and soya).

A protein is made up from a chain of amino-acids. During digestion, the links between these are broken by enzymes, which results in a liberation of amino-acids. All of these are required for our organism to synthesize its own proteins.
Some amino-acids can be produced by the organism, but others can only be supplied by food. The latter are called 'essential amino-acids'. This notion of 'essential amino-acids' establishes the notion of 'biological value' of a protein.
Thus some vegetal proteins have a limited value, i.e. a poor fraction of amino-acids. It is therefore necessary to associate different foodstuffs in one's diet in order to obtain a good biological protein.
Examples of associations

milk + cereals

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'milk-semolina'

cereals + dried vegetables

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'couscous'

cereals + dried fruits

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'tabbouleh'

Concerning animal proteins, they have a good biological value, and they are often found in the giblets or offals or meat.

Food-fats or lipids are essential parts of our body. Half of our brain for instance is made of lipids. However, the need for lipids is not great.
Lipids are useful as a reserve of energy in case of sustained deficiency. Lipids bring us also vitamins A, E and K.
The sources for lipids are either animal (eggs, meats, fishes, cold cuts, butter, cream and cheese) or vegetal (oils, margarine, oleaginous fruits).
Lipids are made of fatty-acids. We can distinguish the saturated fatty-acids and the unsaturated fatty-acids.

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The saturated fatty-acids are present mainly in animal fats, and the unsaturated fatty-acids in vegetable fats. Because of their major biological role, some unsaturated fatty-acids are called essential, but our body is unable to synthesize them itself. They are provided to our organism by food, mainly vegetable oils and fish.

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Saturated fatty-acids are more valuable than the unsaturated type because they keep the cholesterol level down, particularly the 'bad cholesterol', thus protecting our cardio-vascular system.

Energetic value of lipids: 1 g of lipid = 9 calories = 38 KJ

Glucides or carbo-hydrates

Glucides play essentially an energetic role. They are our main source of energy through the provision of glucose, which is the fuel for all cell-types in the human body. For example, glucides are the only source of energy which the brain can utilize, as it is unable to store energy.
Sources for glucose in food are mainly of vegetable origin : leguminous plants, cereals, potatoes (starch), and also fruits, vegetables, sugar beet, and sugar cane. A small fraction of glucides comes from the animal world : milk products and liver (glycogen).
We can separate :

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Rapid sugars: they have a short chain, and are quickly digested, thus rapidly increasing the sugar blood level (glycaemia).

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Low sugars: they have a long chain, and are slowly digested, with a progressive increase of glycaemia.